This invention relates to a control system for regulating the transmission fluid pressure in a transmission, and is particularly useful in maintaining proper pressure control under cold weather conditions when the transmission fluid thickens and its viscosity increases, resulting in a substantially reduced flow rate.
The pressure of transmission fluid is, of course, controlled in a transmission in order to hydraulically actuate appropriate elements to change the ratio between a driving input shaft and a driven output shaft, and this occurs in all types of transmissions, such as industrial, automotive, marine, manual, automatic, continuously variable, etc. In accordance with one well-known pressure control arrangement, pressurized transmission fluid is supplied to an electro-mechanical device, usually a solenoid valve, which is operated by a pulse width modulated (PWM) electrical signal, namely a signal of rectangular waveshape having periodically recurring positive-going pulse components with intervening negative-going pulse components. The frequency of the PWM signal is constant (for example, around 100 hertz) but the relative widths (connoting time durations) of the positive and negative pulse components are varied. As the width or duration of each positive pulse component increases, each negative pulse component decreases proportionally, and vice versa. In other words, since the period or time duration of a complete cycle is fixed or constant, when the duration of a positive pulse component changes in one sense or direction, the width of the immediately succeeding negative pulse component must change in the opposite direction. The PWM signal has a duty cycle characteristic which is the ratio of the width of each positive-going pulse compared to the duration of a complete cycle.
The solenoid valve is turned on and off, or opened and closed, in response to the pulses of the PWM signal to control the flow of the pressurized transmission fluid through the valve. The fluid pressure at the valve's outlet will be established at some level less than that of the pressurized fluid supply as determined by the ratio of the open times relative to the closed times (the greater the ratio, the greater the outlet pressure), which in turn is a function of the PWM signal's duty cycle. When, for example, a higher transmission fluid pressure is required to perform some function (such as to actuate a clutch, to position a sheave, or to shift gears), the duty cycle of the PWM signal is adjusted in the direction and to the extent necessary to produce the desired higher fluid pressure.
Such PWM signal-controlled solenoid valves operate very satisfactorily under normal operating temperatures. Under cold weather or low temperature conditions, however, the transmission fluid may not flow through the valve at a required minimum rate and it may be difficult to obtain proper pressure control. The solenoid will alternately energize and de-energize to open and close the valve, but, due to the increased viscosity and thickness of the transmission fluid at low temperatures, insufficient fluid may flow through the valve during each valve opening and the desired fluid pressure may not be attained. This shortcoming has now been overcome by the present invention which provides a transmission fluid pressure control arrangement that will function appropriately under all weather conditions and regardless of operating temperature. The invention allows a transmission to operate satisfactorily over a much wider temperature range.